Light emitting diodes (LEDs) are solid-state devices that convert electrical energy to light, and generally comprise one or more active regions of semiconductor material formed between oppositely doped materials. When a bias is applied across the doped materials, the active region generates light that can be emitted from all surfaces of the LED. In addition to LEDs, solid-state lighting (“SSL”) devices can use organic light emitting diodes (“OLEDs”), and/or polymer light emitting diodes (“PLEDs”) as sources of illumination, rather than electrical filaments, plasma, or gas. SSL devices are used in a wide variety of products and applications including common consumer electronic devices such as mobile phones, personal digital assistants (“PDAs”), digital cameras, MP3 players, and other portable electronic devices utilize SSL devices for backlighting. SSL devices are also used for traffic lighting, signage, indoor lighting, outdoor lighting, and other types of general illumination.
In many applications, it is desirable to have SSL devices that provide high light output with better performances being realized through reducing the difference between power supply output voltage and input voltage. One conventional technique of achieving high input voltage in LEDs is serially coupling a plurality of LED dies in an array. In certain embodiments, the individual SSL dies may include more than one LED junction coupled in series.
FIG. 1A is a cross-sectional view and FIG. 1B is a top plan view of a conventional high voltage SSL device 10 shown with two junctions in series. As shown in FIGS. 1A and 1B, the high voltage SSL device 10 includes a substrate 20 carrying a plurality of LED structures 11 (identified individually as first and second LED structures 11a, 11b) that are electrically isolated from one another by an insulating material 12. Each LED structure 11a, 11b has an active region 14, e.g., containing gallium nitride/indium gallium nitride (GaN/InGaN) multiple quantum wells (“MQWs”), positioned between P-type GaN 15 and N-type GaN 16 doped materials. The high voltage SSL device 10 also includes a first contact 17 on the P-type GaN 15 and a second contact 19 on the N-type GaN 16 in a lateral configuration. The individual SSL structures 11a, 11b are separated by a notch 22 through which a portion of the N-type GaN 16 is exposed. An interconnect 24 electrically connects the two adjacent SSL structures 11a, 11b in series through the notch 22.
In operation, electrical power is provided to the SSL device 10 via the contacts 17, 19, causing the active region 14 to emit light. Higher light output can be achieved at the assembly level by mounting several high voltage SSL devices onto a single circuit board, e.g., an LED package array to deliver higher flux. Typical arrays include many LED packages which can be coupled in series, in parallel or in a combination of series and parallel coupled packages. For example, high voltage can be achieved by wiring several conventional high voltage SSL devices 10 in parallel configuration. Arrays of high voltage SSL devices can be advantageous in that the number of LED packages included in the array is independent of the total package voltage (U.S. Patent Publication No. 2012/0161161, incorporated herein by reference in its entirety). However, despite improved light output and higher flux delivery, arrays incorporating the SSL device 10 of FIGS. 1A and 1B are subject to junction failure which can cause problems with chip usability, deterioration, and create high variation in bias across individual coupled SSL devices in the array. For example, an individual LED structure 11a can fail, become an open circuit, or become a short circuit, causing the remaining LED structure 11b as well as other serially or parallel coupled dies to fail, reduce performance or lose stability. Accordingly, there remains a need for high voltage LEDs, high voltage LED arrays and other solid-state devices that facilitate packaging and have improved performance and reliability.